This invention relates to improvements in an opto-magnetic recording medium which is used in an opto-magnetic memory, a magnetic recording and displaying element or the like and which can be read out by the use of the magneto-optical effect such as the magnetic Kerr effect or the Faraday effect.
As the recording layer of an opto-magnetic recording medium, there is known a polycrystal thin film such as MnBi or MnCuBi, an amorphous thin film such as GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo, TbFeCo or GdTbCo, or a single crystal thin film such as GdlG.
Of these thin films, the amorphous thin film is recently considered excellent as the recording layer of the opto-magnetic recording medium with the film formability with which a thin film of large area is made at a temperature in the vicinity of room, temperature, the writing-in efficiency for writing signals at small opto-thermal energy and the readingout efficiency for reading out the written signals at a good S/N ratio being taken into account. Particularly, GdTbFe, which has a great Kerr rotation angle and a curie point of about 150.degree. C., is best suited as the material for the opto-magnetic medium.
However, amorphous magnetic materials including GdTbFe have a disadvantage that they are generally inferior in corrosion resistance and tend to be corroded in an atmosphere having moisture and their magnetic characteristic is deteriorated. If an element such as Si, Cr or Ti is added to the amorphous magnetic material to overcome such disadvantage, the corrosion resistance will be improved, but instead there will occur a disadvantage that the curie point is elevated and the recording sensitivity is reduced.
To eliminate such disadvantage, there have heretofore been proposed disk-like opto-magnetic recording mediums of the type in which various protective layers are provided on a recording magnetic layer of amorphous magnetic material, or of the air sandwich structure in which the recording magnetic layer is enveloped by inert gas, or of the cemented structure in which a substrate is further provided on the recording magnetic layer with an adhesive agent interposed therebetween.
As said protective layer, there is, for example, a thin film of an oxide such as SiO.sub.2, SiO or Al.sub.2 O.sub.3, a thin film of a metal such as Ti, Cr, Al or Si, or a thin film of a nitride such as AlN or Si.sub.3 N.sub.4.
However, where the magnetic recording layer is as thin as up to several hundred .ANG., the effect of improving the durability by a thin film of oxide is not sufficient. Also, a thin film of nitride is inferior to oxides in the adhesion property with respect to the substrate, particularly plastic substrate, and has a further disadvantage that it is too high in refractive index.
It is an object of the present invention to provide an opto-magnetic recording medium using an amorphous magnetic material which is improved in corrosion resistance and durability and in which the writing-in efficiency and the reading-out efficiency can also be improved.
Such object is achieved by an opto-magnetic recording medium having on a substrate at least a recording layer consisting of an amorphous magnetic material and wherein an under-coated layer comprising a silicon oxide film, a metal film and a nitride film laminated in succession from the substrate side is provided between said substrate and said recording layer.
The present invention is particularly suitable for a case where said substrate is plastic.
Also, in the present invention, the silicon oxide film is an SiO or SiO.sub.2 film. As the metal used in the metal film, mention may be made of Si, Cr, Ni, Ti, Al or the like. As the nitride forming the nitride film, mention may be made of Si.sub.3 N.sub.4, AlN, BN or the like.
By the use of the above-mentioned materials, the silicon oxide film is formed to a thickness of 500.ANG.-2000.ANG., preferably of 500.ANG.-1000.ANG., the metal film is formed to a thickness of 20.ANG.-100.ANG., preferably of 20.ANG.-40.ANG., and the nitride film is formed to a thickness of 200.ANG.-1000.ANG., preferably of 200.ANG.-400.ANG..
The forming method may be sputtering, vacuum evaporation or the like.